Hi Ken,
Digging around a bit;
The NDS commentary available in the download above(section 3.2.3.1) says;
Notches are a special problem in bending members due to the stress concentrations occuring at the corners and the difficulty of calculating the effects of shear and perpendicular to grain stresses occuring at such locations. These stress concentrations can be reduced by using gradually tapered rather than square cornered notches.

That comment references the Forest Products Labs "Wood Handbook"
Downloading chapter 8;
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch08.pdf
The reference seems to be referring to the section beginning at the bottom of 8-6 and continuing onto the next page.

References 91,92,132 in the commentary look promising also.

I remember a comment either in the NDS or the codebook I think stating that a gradually tapered notch reduces shear stress to nearly unnotched levels.
..found it;
"3.4.3(4) A gradual change in cross section compared with a square cornered notch decreases the actual shear stress parallel to grain nearly to that computed for an unnotched bending member with a depth of Dn (the remaining depth)

I also did a quick search on the FPL site and got 28 hits on the word "notch" It doesn't take much for those boys to go over this carpenters head. They do perform testing there, they drive the NDS and our codes and do respond to mail as well.

Also to expand on Andrea's comment. The 1/4 depth is for end notching, 1/6 depth on compression edge of heavy timber in outer thirds of span, none in center third. In glulam 1/10 of depth or 3" whichever is less for ends, none elsewhere.

I'm sure not an engineer though.